As filter media for air filter meeting the criteria of an HEPA filter (High Efficiency Particulate Air Filter), filter media produced by using glass fibers called glass fiber filter media are known. While glass filter media of HEPA grade have a high collection efficiency, i.e., a collection efficiency of 99.97% for particles having a particle diameter of 0.3 μm, their pressure loss is high.
As HEPA filters with low pressure loss in place of such glass filter media, filter media using polytetrafluoroethylene (PTFE) porous film produced by stretching PTFE are known. Compared to glass filter media, PTFE porous films have high collection efficiency and low pressure loss, and excel in balancing collection efficiency and pressure loss.
For example, each of the following Patent Literature 1 (Japanese Laid-open Patent Publication No. 2013-52320) and Patent Literature 2 (Japanese Laid-open Patent Publication No. 2013-63424) proposes a filter medium for an air filter equipped with a melt-blown nonwoven fabric on the upstream side of a PTFE porous film, and further equipped with an air-permeable cover layer composed of a spunbond nonwoven fabric on the upstream side of the melt-blown nonwoven fabric, to reduce pressure loss of an air filter unit used while retained by a shape retaining member.